A new geochemical approach for constraining a marine redox condition of Early Archean

This paper presents a new approach for determining the paleo-redox conditions of the Early Archean deep-sea using U–Th systematics of seafloor hydrothermally altered basalts. In the modern ocean system, fresh unaltered basaltic glasses from the seafloor exhibit a tight U–Th correlation, which reflects magmatic processes. However, basalts that have been hydrothermally altered are noticeably enriched in U due to adsorption and precipitation of dissolved U from oxygenated deep-sea water onto the altered rocks. Irrespective of alteration temperature they exhibit much higher U/Th (up to 34) than the primary igneous ratio of ∼0.35. In marked contrast, hydrothermally altered basalts from the Early Archean (3.46 Ga) seafloor have maintained igneous U/Th (∼0.25) perfectly, in spite of having been subjected to intense alteration. This can be explained by a lack of dissolved U in Early Archean deep-sea water due to the prevalence of anoxic deep-sea conditions.